This application relates to a new form of zeolite beta and to its use as a catalyst in the alkylation of aromatics. More particularly, this application relates to a zeolite beta which shows substantially greater selectivity when used in the alkylation of benzene by ethylene. It is contemplated that the catalyst of this invention will be particularly valuable in production of high purity ethylbenzene, in minimizing formation of the diphenylethane which accompanies benzene alkylation by ethylene and in maximizing benzene utilization.
Ethylbenzene is the major article of commerce which is commonly made by the alkylation of benzene with ethylene. As is usual, several byproducts accompany ethylbenzene formation; a simplified summary of alkylation processes and products commonly occurring are given below. ##STR1## Zeolite beta has been found to be an effective catalyst and has gained a prominent role in the alkylation of benzene by ethylene. Although the formation of isomeric diethylbenzenes and triethylbenzenes might, at first glance, be viewed as byproducts representing a loss of ethylene, hence a reduction in efficiency of ethylene utilization, in fact each can be readily transalklylated to afford ethylbenzene as the sole alkylated benzene. ##STR2## In contrast, diphenylethane can not be converted to ethylbenzene via alkylation and thus represents a loss of ethylene and a reduction in ethylene utilization efficiency. In fact, the coproduction of diphenylethane and polyalkylated benzenes, where the latter are collectively known as heavies, represents virtually all of the reduction in ethylene utilization.
Where Y zeolite is used as a catalyst in the reaction of ethylene and benzene approximately 0.65% DPE and about 0.55 weight percent of heavies are formed, resulting in a total loss of about 1.2%. Where zeolite beta is used only about 0.4% DPE and about 0.1% heavies are formed, resulting in a loss of 0.5%. Although this improvement is small it also is very significant, resulting in zeolite beta gaining favor as a catalyst of choice for ethylbenzene production. However, formation of even the latter small amount of DPE and heavies is vexing and gave impetus to further research whose goal was to reduce losses still further.
In the course of these investigations it was observed that a zeolite beta subjected to a carbon burn, or conditions of a carbon burn, afforded product with a significant reduction in DPE content. This result not only was serendipitous and surprising but also was not predictable from any known property of zeolite beta. This result was reproducible and forms the basis for our invention. A process of alkylating benzene with ethylene using the catalyst of this invention shows a significant selectivity advantage over one using untreated zeolite beta as the catalyst.
Our working hypothesis for the underlying chemistry responsible for the observed results is that when zeolite beta is subjected to a carbon burn, or conditions of a carbon burn, there is a selective loss of sites effecting hydride transfer reactions. Consequently, such a modified zeolite beta leads to reduced hydride transfer and products associated with hydride transfer. One general class of such products is the formation of diarylalkanes as a byproduct of virtually all aromatic alkylations by olefins. Thus, in the alkylation of benzene with ethylene there is reduced formation of diphenylethane. Another general class of such products are the n-alkylaromatics formed in the alkylation of aromatics with olefins. This is exemplified by a reduction in n-propylbenze formation accompanying alkylation of benzene by propylene. Yet another result which can be anticipated from our hypothesis is a reduction in phenylcyclohexane formation accompanying any alkylation of benzene. Cyclohexane is a common impurity in benzene, and hydride transfer during benzene alkylation can lead to formation of cyclohexene. The latter serves as an active alkylating agent and reacts with benzene to form phenylcyclohexane. The outcome of the foregoing hypothesis and its logical consequences is that one can expect the catalyst of this invention with reduced hydride transfer sites to confer benefits generally upon the alkylation of aromatics with olefins.
Another outcome of our hypothesis is that any method leading to a reduction in hydride transfer sites of a zeolite beta affords material useful in the process of our invention. Defining for the purpose of this invention a "site-modified zeolite beta" as one which has fewer hydride transfer sites than a native, untreated zeolite beta, clearly effecting a carbon burn is but one method of producing such a site-modified zeolite beta. Another method of achieving similar results is to calcine zeolite beta for extended times in a steam atmosphere at a temperature in excess of 675.degree. C.
Ratcliffe, U.S. Pat. No. 4,876,408, previously has used a carbon burn for several zeolites, including zeolite beta, to modify the catalyst so as to increase its selectivity for monoalkylation by at least 1.0 percentage point. On contrast to the Ratcliffe catalyst, our site-modified zeolite beta decreases monoalkylation selectivity, an observation which is certainly unexpected in view of the contrary prior art teaching| It also is not obvious that a decrease in monoalkylation would be desirable, or even tolerable. However, as we discussed earlier, we recognize that the critical feature in alkylation is minimizing diphenylethane formation, which our catalyst accomplishes more effectively than the prior art catalysts.
Although Shamshoum et al. have taught a steam-modified zeolite beta for aromatic alkylation in U.S. Pat. No. 5,227,558, their catalyst is functionally quite distinct from ours, as will become apparent in our presentation of experimental data. The patentees' catalyst also has a significantly higher ratio of silica to alumina than the catalysts taught herein--and outside the range we believe important to the success of our invention|.